1. Field of the Invention
The present invention relates to the field of electrophotographic imaging and particularly to the field of electrophotographic imaging on transparent surfaces that are used to project images, as with overhead transparencies.
2. Background of the Art
Electrophotography forms the technical basis for various well-known imaging processes, including photocopying and some forms of laser printing. Other imaging processes use electrostatic or ionographic printing. Electrostatic printing is printing where a dielectric receptor or substrate is “written” upon imagewise by a charged stylus, leaving a latent electrostatic image on the surface of the dielectric recepetor. This dielectric receptor is not photosensitive and is generally not re-useable. Once the image pattern has been “written” onto the dielectric receptor in the form of an electrostatic charge pattern of positive or negative polarity, oppositely charged toner particles are applied to the dielectric receptor in order to develop the latent image. An exemplary electrostatic imaging process is described in U.S. Pat. No. 5,176,974.
In contrast, electrophotographic imaging processes typically involve the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, and cleaning, and erasure.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a corona or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent image by selectively exposing the photoreceptor to electromagnetic radiation, thereby discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. The electromagnetic radiation, which may also be referred to as “light”, may include infrared radiation, visible light, and ultraviolet radiation, for example.
In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using a developer electrically-biased to a potential of the same polarity as the toner polarity. The toner particles are more strongly attracted to the discharged regions of the photoreceptor and migrate to the photoreceptor and selectively adhere to the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor; an intermediate transfer element is sometimes used to effect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor. The transfer of an image typically occurs by one of the following two methods: elastomeric assist (also referred to herein as “adhesive transfer”) or electrostatic assist (also referred to herein as “electrostatic transfer”).
Elastomeric assist or adhesive transfer refers generally to a process in which the transfer of an image is primarily caused by balancing the relative energies between the ink, a photoreceptor surface and a temporary carrier surface or medium for the toner. The effectiveness of such elastomeric assist or adhesive transfer is controlled by several variables including surface energy, temperature, pressure, and toner rheology. An exemplary elastomeric assist/adhesive image transfer process is described in U.S. Pat. No. 5,916,718.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically nonconductive dispersant, to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated to some degree in the liquid carrier (or carrier liquid), typically in more than 50 weight percent of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles. Because of their small particle size, ranging from about 5 microns to sub-micron, liquid toners are capable of producing very high-resolution toned images. This distinguishes dry toner particles from liquid toner particles.
A typical toner particle for a liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The polymeric binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g. glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g. temperature, pressure and fuser configuration) impact durability (e.g. blocking and erasure resistance), adhesion to the receptor, gloss, and the like.
Polymeric binder materials suitable for use in liquid toner particles typically exhibit glass transition temperatures of about −24° C. to 75° C., which is lower than the range of glass transition temperatures (50–100° C.) typical for polymeric binders used in dry toner particles. In particular, some liquid toners are known to incorporate polymeric binders exhibiting glass transition temperatures (Tg) below room temperature (25° C.) in order to rapidly self fix, e.g., by film formation, in the liquid electrophotographic imaging process; see e.g. U.S. Pat. No. 6,255,363. However, such liquid toners are also known to exhibit inferior image durability resulting from the low Tg (e.g. poor blocking and erasure resistance) after fusing the toned image to a final image receptor.
In other printing processes using liquid toners, self-fixing is not required. In such a system, the image developed on the photoconductive surface is transferred to an intermediate transfer belt (“ITB”) or intermediate transfer member (“ITM”) or directly to a print medium without film formation at this stage. See, for example, U.S. Pat. Nos. 5,410,392 to Landa, issued on Apr. 25, 1995; and U.S. Pat. No. 5,115,277 to Camis, issued on May 19, 1992. In such a system, this transfer of discrete toner particles in image form is carried out using a combination of mechanical forces, electrostatic forces, and thermal energy. In the system particularly described in the '277 patent, DC bias voltage is connected to an inner sleeve member to develop electrostatic forces at the surface of the print medium for assisting in the efficient transfer of color images.
Since the introduction of electrophotographic copying and printing machines using dry toner powder particles to develop electrostatic images, there has been a continuing emphasis on toner image transfer with faithful, high quality, durable fused image reproduction on the surface of a receptor sheet. Previous studies related to the quality of images produced by transfer of toner powder, from imaging drums of electrophotographic copiers and printers to suitable recording sheets, focused attention on the bond formed between the powder and the recording sheet. Having demonstrated sufficient adhesion, measurement of optical density indicated the intensity of the image formed on the recording sheet, as shown by U.S. Pat. No. 5,451,458. U.S. Pat. No. 5,302,439 discloses a recording sheet which comprises a substrate and a coating thereon Materials from the various groups increase the adhesion of toner powder to the recording sheet.
More recently, attention has turned to improvement of the projected transparency of images formed by fusing dry toner particles to transparent receptor films, Poor transparency of fused dry toner images is believed to result from multiple light scattering from solid particles (e.g. pigment particles) having a volume mean diameter generally larger than approximately 0.5–1 micron, such that light projected through the fused toner layer undergoes multiple scattering from the solid particles. Alternatively, multiple light scattering may occur at the surfaces and edges of the fused transparency image when the projector's light source is transmitted through the fused toned image on the transparency receptor.
The problem of light scattering is particularly undesirable and tends to be more noticeable with the use of colored pigments. In the use of black and white images, light scatter tends to only cause increased opacity or increased transmission optical density of the projected image, which is minimally problematic, as the projected image actually appears more black. However, when light is scattered by multi-colored pigment particles, there is a color shift effected, and this results in not only altering the transmitted optical density and projected image opacity, but also the color content itself. The imaging process must retain the ability to provide high fidelity to the intended colors and high color quality when the fused toned image on the receptor film is projected, or the image quality will be seriously degraded. For example, one of the most apparent scattering effects is noted in overhead projection of toned yellow colors, where the projected yellow image is often “muddy,” appearing as gold, light green, brown or even black, depending upon smoothness of the fused toner film and the extent of light scattering from both the surface and the volume of the fused toned image.
Various approaches have been used in the art to improve the projected transparency of fused dry toner images. One approach involves alteration of the toner composition to improve uniformity of the fused toner layer on the transparent image receptor. U.S. Pat. No. 5,635,325 discloses a core/shell toner for developing electrostatic images including a binder resin, a colorant and an ester wax, wherein the core melts and acts as a release agent during fusing, eliminating the need for silicone based release agents to be applied to the fuser rolls.
Other approaches to improve the projection quality of fused dry toner images on transparency receptors involve use of special coatings on the transparency receptor to improve coalescence of the toner powders into a smooth, uniform layer on the receptor. U.S. Pat. Nos. 5,208,093, 4,298,309 and 5,635,325 disclose a variety of solutions to achieve miscibility of the coated film with the toner while maintaining low melt viscosity. U.S. Pat. No. 5,451,458 discloses a recording sheet which comprises a substrate and a coating thereon containing a binder selected from polyesters, polyvinyl acetals, vinyl alcohol-vinyl acetal copolymers, polycarbonates, and mixtures thereof, and an additive having a melting point of less than about 65 degree C. and a boiling point of more than about 150 degree C. U.S. Pat. Nos. 6,391,954 and 6,296,931 describe a recording sheet including an additive, referred to as a compatibilizer, to improve the quality of images formed by toner powder development of electrostatic charge patterns.
Still other approaches are directed at improved toner transparency using special fixing methods for fusing the dry toner powder to the receptor sheet. U.S. Pat. No. 5,824,442 describes the use of special toners in such a fixing method. U.S. Pat. No. 5,519,479 describes a fusing or fixing device for use in an electrophotographic apparatus, comprising a pair of pressing means opposing each other to form therebetween a nip through which an image supporting member supporting an unfixed toner image is passed so that the toner image is fixed to the image supporting member, wherein one of the pressing means which contacts the toner image on the image supporting member has a layer formed of a soft matrix and granular particles dispersed in the matrix and having greater hardness than the matrix, whereby fine irregularities are formed on toner surfaces on the image supporting member by the granular particles under application of pressure during fixing. This is clearly contraindicated as a solution against light scattering that shifts color balance and fidelity.
For the case of fused liquid toner images on transparent receptor sheets, all of the previously described problems may occur. Poor fused toner adhesion, unacceptable projected image transparency due to light scattering by oversized pigment particles or surface irregularities in the fused toner layer, and poor color fidelity in multi-color fused toner images remain problematic. In addition, the difficulty of producing durable, transparent, multi-colored fused liquid toner images on transparency receptors is compounded by the fact that a substantial amount of carrier liquid is typically present in the toned image prior to fusing on the final transparency receptor. The latter issue is particularly problematic for transparencies produced using liquid electrophotographic imaging processes that make use of an electrostatic transfer assist to transfer the toned image to the final image receptor, because a substantial amount of carrier liquid is required in the toned image in order to effect electrostatic transfer. This carrier liquid may have adverse effects on the durability of the fused liquid toner image if it remains in the fused image. Alternatively, removal of the carrier liquid may have adverse effects on the transparency and color fidelity of the projected liquid toned image.
Copending U.S. patent application Ser. No. 10/750,458, filed Dec. 31, 2003, titled “REDUCED LIGHT SCATTERING IN PROJECTED IMAGES FORMED FROM ELECTROGRAPHIC TONERS” discloses a method for improving the light-scattering properties of electrographic toned images used in transparencies. That application is incorporated herein by reference. It is still necessary to identify, measure and otherwise evaluate the quality of projected transparency images to determine how and when the image or the projection facility must be modified or improved.
Optical systems have been used in many varied fields of technology for measuring conditions and properties. The optical systems can be extremely simple, measuring the required or unwanted presence of light or radiation (e.g., Ultraviolet light, visible light, or infrared radiation), or by more complex systems that determine and/or measure the properties of light at specific locations (including wavelengths, phases, intensity, patterns and the like). A grating spectrometer is an example of a complex optical staring] spectrum analyser. This works by splitting the input beam into many hundreds of beams, changing the phase of each beam by an amount which depends linearly on its position (using the grating) and recombining all of the phase shifted beams on an output detector array. Because of the phase shifts, different optical frequencies recombine in phase at different places in the detector array.
Another type of staring optical spectrum analyser is an acoustic-optic device in which the signal to be analysed is used to drive an acoustic-optic transducer which launches an acoustic wave into a transparent piezoelectric and electro-optic material (e.g. lithium niobate). The acoustic waves can set up refractive index waves in such materials which diffract a light beam passing through them by an amount directly proportional to the RF frequency. In practice, this type of spectrum analyser can give very high resolution, mainly because acoustic waves travel much more slowly than electromagnetic waves, allowing longer delays to be achieved in short devices. However, they tend to be limited to frequencies below a few GHz because of acoustic losses.
U.S. Pat. No. 5,132,627 (Popovic et al.) teaches a motionless scanner for use in electrophothgraphy. In its disclosure, a beam splitter is used in certain instances. It is disclosed that exposure light is transmitted through an electrode 16 to photoreceptor 10. For tests which require on-line monitoring of exposure light intensity a beam splitter 32 deflects a portion of the illumination light to a photodiode 33. Coulomb meter 29a is used in two ways, either to measure charge flow through the photoreceptor sample or to monitor the illumination light energy by measuring the charge flow through photodiode 33. Most instruments such as electrostatic meter 28, coulomb meter 29a, exposure light source 31 and high voltage supply 20 are connected directly to the data acquisition board of computer 30, but others such as relay 24 utilize simple interface circuitry.
U.S. Pat. No. 5,543,177 (Morrison et al.) teaches marking materials containing retroreflecting fillers describes the use of retroreflectors as timing marks on a photoreceptor belt used in xerographic copiers and printers was simulated as follows. In a typical prior art machine, the standard timing marks on belt photoreceptors are rectangular holes in the opaque ground strip. A light source is put on one side of the belt, a detector on the other. The time at which the timing hole passes can be determined by situating a light source on one side of the belts and a detector on the other. (Another method to detect the hole is to use an infrared source to illuminate the opaque ground strip as it moves.) The illuminating beam comes from a nanometer wavelength light emitting diode. By passing the beam through a beam splitter, the light reflected off the conductive ground plane can be detected at nearly 90 degree specular reflectance. Such a device is a Xerox® 1075 CIRD (part number 130S941, Xerox Corp., Rochester, N.Y.). If the illumination falls on the timing hole, no light is reflected.
U.S. Pat. No. 6,668,104 (Mueller-Fiedler et al.) describes a method for detecting the wetting of a surface, for example, a windshield on a vehicle, using optical sensing. A beam splitter is suggested as one format for providing the optical (including infrared) beam. Light from the visible range or the infrared range is coupled into the windshield from the inside of the windshield. The unmoistened outer surface reflects the light, which reaches a receiver. To increase the efficiency, the light is shone in in such a way that total reflection takes place on the outside. The total reflection is disturbed by the wetting of the outer surface with water. It is a common feature of all the known versions that the input and output of the electromagnetic waves take place at spatially markedly separate points, and that the sensor element and the evaluation electronics are accommodated in a common housing. Error-free signal detection can then be accomplished only if the optical sensor is mounted in a region of the windshield that is cleaned by the windshield wiper system. Therefore in some vehicle types, the sensor has to be mounted at a distance of up to 15 cm from the upper edge of the windshield. A disadvantage of this is that the sensor housing in these cases is within the field of view of the driver and is perceived as annoying because of the lack of transparency. Miniaturization is not possible, since for timely detection of wetting, for instance when it is beginning to rain, a sensor region approximately 4–5 cm2 in area is necessary.
U.S. Pat. No. 6,570,840 (Wilkinson et al.) discloses improvements in the transverse sectional shape of three-dimensional features displayed in optical recording structures—discs, cylinders, cards, multi-layered devices and structures replicated from them—to increase Figure of Merit. The cross-sectional shape improvements include reductions in berm height and width, dual level data marks and tracking guides, and land areas projecting above or into the surface of the recording structure. Disclosed methods include dual and/or dithered beam writing onto the structure and improved composition of the active layer of the structure. Also disclosed are apparatus for producing such improved features on optical recording structures, according to one or more of the disclosed methods. In one embodiment, a dithered secondary beam whose intensity is greater, and whose focused spot size is narrower, than the secondary beam configuration previously described. In this embodiment, the secondary beam is dithered—oscillated rapidly in a radial direction, in respect to the disc—while trough formation occurs. A variation of the latter embodiment comprises use of beam splitting to produce a plurality of overlapping, side-by-side non-dithering beams whose sum is a beam of essentially uniform intensity along all or a portion of its width. A further variation comprises beam dithering without beam splitting, where the single beam executes a complex dithering motion that is radial in respect to the disc (i.e., transverse, in respect to the longitudinal dimension of the formed features), and thus may produce pits and a trough simultaneously.
U.S. Pat. No. 6,055,391 (Jackson et al.) describes a system for detecting and damping vibrations in printer components, mechanical devices, buildings, or large structures has a plurality of light beam detectors for generating signals corresponding to a reference light beam position. The light beam detectors are partially transparent to allow passage of the light beam through the detector, permitting multiple detectors to use the same reference light beam. The detectors are attached to vibration susceptible structural elements, with detected movement of the light beam with respect to the partially transparent light beam detector corresponding to movement of the vibration susceptible structural element. Motion control units connected to the detectors can be used to control or damp detected vibrations in real-time. The printer system may have a vibration detection and suppression unit mounted on the inside of a panel. The unit may include a laser light beam source that directs a light beam to pass through a number of partially transparent light beam detectors distributed along the panel. In one embodiment, a beam splitter and mirrors are used to redirect the light beam to those detectors not in direct line with the laser light beam source. Other light beam redirecting or bending systems can be used, including light scanners, polygon scanners, fiber optics, or prisms. For certain applications, design of detectors capable of refractive bending of light to redirect a beam pass through is contemplated (e.g. prism detectors). If information regarding light edge position, rather than light spot position is adequate (e.g. in conjunction with one dimensional detectors), various light diffusers or spreaders can also be used.
U.S. Pat. No. 5,790,255 describes a system of partially transparent light beam detectors that allow passage of the light beam through the detector allows multiple detectors that use the same reference light beam. Transparent detectors can be position sensitive detectors (PSD), photodiode arrays, or CCD imaging arrays. The detectors are attached to vibration susceptible structural elements, with detected movement of the light beam with respect to the partially transparent light beam detectors corresponding to movement of the vibration susceptible structural element.
It is desirable in the improvement of transparency images to have a system that can actually determine the quality of the image under projection conditions without having to project the image on a screen and make a qualitative or visual interpretation of the quality.